This is a proposal to develop a high-resolution combined single photon (SPECT) and positron emission tomograph (PET). The dual PET/SPECT scanner will have excellent spatial resolution and sensitivity making quantitative analysis of in vivo functional animal imaging possible. This scanner will greatly facilitate the development and evaluation of imaging agents and pharmaceuticals that are used for a broad variety of diagnostic, therapeutic, therapy monitoring and experimental purposes. The medical fields that benefit from nuclear emission imaging include cardiology, endocrinology, hematology, oncology, osteology, neurology and genetic manipulation and modeling. The PET/SPECT offers great potential to more effectively evaluate the efficacy of agents by replacing current ex vivo, time consuming and expensive methods with more accurate tests in live rodent models. The PET/SPEC utilizes GSO scintillators that provide the necessary performance for both PET and SPECT which are traditionally separate machines. Recent efforts to combine PET and SPECT on human scanners are widespread. Likewise, there is great interest in animal PET and SPECT. This is the first commercial effort to combine PET and SPECT for animals. An extremely versatile electronics processing and data handling scheme will combine with advanced technology scintillators and photosensors to meet the challenge of combining two imaging modalities with very different performance demands. During Phase I we will assemble 1-inch prototypes of the system detectors and characterize the single photon (at 140 keV) and coincidence (511 keV) response in terms of detector efficiency. Energy resolution, timing resolution, and other important characteristics as a proof of feasibility for the full system will be developed in Phase II.